In current civil engineering and building construction practice, many structures ranging from residential houses to high-rise buildings are founded on deep foundation systems, such as piles or drilled piers, which extend to rock or stronger soils to support the building. This is often necessary because soils near the surface frequently are inadequate for supporting the building upon a shallow foundation. These deep foundations tend to be rather expensive compared to shallow foundations and are typically necessary where the near-surface soils include soft to stiff clays, silts, sandy silts, loose to firm silty sands and sands. In most shallow foundations, the amount of settlement tolerable (influenced by the soil's compressibility) controls the usefulness of the shallow foundation, rather than the ultimate load-bearing capacity (strength). For some situations where the near-surface soils are inadequate or marginal for supporting shallow foundations, the in situ soils can be stiffened with reinforcement, such as short aggregate piers. This allows shallow foundations to be used in place of deep foundations or smaller footings to be used in circumstances where space limitations are critical. In either instance, a substantial cost savings can be realized using short aggregate piers to reinforce the near-surface soils.
Similar improvements in subgrade, subbase and base materials beneath highways, railroads, and runways can result in substantial savings in construction costs. For example, in most highways that are built in non-fill soil sites, the in situ soil is incapable of adequately supporting a thin pavement wearing surface. The traditional solution is to excavate the existing soil to a certain depth, usually between four (4) and twenty-four (24) inches and replace the removed material with a material having greater load-bearing capabilities in a combination of compacted subbase and base layers. As an alternative, the thickness of the wearing surface can be increased to reduce potential damage from traffic caused by the poor load-bearing characteristics of the subgrade soil. In either event, a substantial cost is associated with the excavation and replacement or with the increased thickness of the wearing surface.
There are two well-known methods for producing a type of deep soil reinforcement known commonly as "stone columns" in situ to strengthen soft soils. These two methods are the so-called "vibro-replacement" and the "vibro-displacement" methods. These methods have been successfully used in numerous major projects in which the in situ soil was primarily soft clay. Each of these methods produces an improvement in the load-bearing capability of the ground, rather than producing a piling resting on bedrock, although stone columns are relatively deep and are often extended to stronger subsoils or even to bedrock.
The vibro-replacement technique (also known as the "wet method") involves jetting a hole into the ground to a desired depth using a vibratory probe (Vibroflot). The jetting is normally accomplished by forcing liquid under great pressure through a lower end of the probe to loosen and cut the soil and by forcing the probe downwardly into the ground. The uncased hole is then flushed out and, typically, uniform graded stone (stone which has been graded to have a relatively uniform particle size) is placed in the bottom of the hole in increments and is compacted by raising and lowering the probe, while at the same time vibrating the probe. The vibro-replacement method is characterized by relatively high cost owing to the rather heavy and specialized nature of the equipment necessary to carry out the method. This has tended to limit the use of the method to relatively large and expensive projects. Also, this technique can have a negative impact on the local environment due to the large quantities of water that are typically used in the process. This causes difficulties in disposing of the excess water and typically results in pools of standing water collected near the constructed columns. These pools of water can impede construction efforts at the site, and add additional cost to the construction.
The second of the above-identified common methods of producing relatively deep stone columns in the ground is known as the "vibro-displacement" or dry method. In the vibro-displacement method, a vibratory probe is forced downwardly into the ground, displacing soil by compaction downwardly and laterally. Compressed air is forced through the tip of the probe to ease penetration into the ground. Once the probe has reached the desired depth, which typically is 20 to 50 feet, the probe is withdrawn and backfill is added to the hole, the backfill typically being drawn from the site itself. The backfill is then compacted using the probe. Several iterations of the filling and compacting steps typically are required to produce a deep stone column having improved load-bearing characteristics as compared with the naturally occurring surrounding soil. The vibro-displacement method also suffers from requiring heavy specialized construction equipment and is generally best suited for improving firmer soils that have a deep groundwater table.
Each of the above-described methods for creating deep stone columns or granular columns, and other known techniques for producing stone or granular columns in relatively soft soils, generally fails to fully exploit the increased load-bearing capacity of the soil surrounding the stone columns if the soil were to be significantly prestressed and densified, as by high energy lateral impact stress. This failure to laterally prestress or compact the surrounding soil to a significant degree is noteworthy because such stone or granular columns are cohesionless, and while being stiffer than the surrounding soil, the columns derive much of their load-bearing capability from the surrounding lateral soil.
Accordingly, it can be seen that a need yet remains for a method of producing reinforcing elements in situ in soils wherein the surrounding lateral soil adjacent the resulting reinforcing elements are significantly prestressed and compacted to improve the load-bearing capability of the reinforcing element, while at the same time being capable of being carried out with relatively inexpensive and simple equipment. It is the provision of such a method, and the reinforcing elements that result therefrom, that the present invention is primarily directed.